The invention relates to conductive pigments having a partly modified surface.
Conductive pigments are usually metal powders, non-metallic powders, such as carbon black and doped metal oxides, or pulverulent carrier materials coated with conductive layers. The first group includes, for example, silver powder.
Examples of doped metal oxides are antimony-doped tin oxide, halogen-doped tin oxide or aluminum-doped zinc oxide, which are described in U.S. Pat. No. 4,655,966, EP 0 441 427 and U.S. Pat. No. 5,171,364.
Carrier materials which are used for the third group of conductive pigments are, inter alia, phyllosilicates, in particular mica, silicon dioxide, titanium dioxide and barium sulfate. These carrier materials are in general coated with a conductive layer of doped metal oxides. DE 40 17 044 describes an electrically conductive barium sulfate, the conductive layer of which comprises antimony-doped tin oxide. A conductive pigment comprising a mica and antimony-doped tin oxide is described in EP 0 139 557. Hollow beads of silicon dioxide coated with antimony-doped tin oxide are known from EP 0 359 569.
If the conductive pigments mentioned are incorporated into a non-conductive matrix, conductive composite materials are only obtained if the concentration in the total matrix lies above a critical concentration range at which the first conductivity pathways can form. This concentration range depends on the pigment and matrix. This range is also called the percolation threshold. It depends chiefly on the geometric shape and the density of the particles.
Conductive pigments are useful for addition to non-conductive materials, such as plastics, in order to make them antistatic or to make them conductive. When made antistatic, the materials do not conduct electric current; however, they prevent the accumulation of electrostatic charges because such charges can flow off the materials. The antistatic region lies between 106 and 109 ohms. At conductance above this antistatic region, the material is an insulator and, at conductance below this region, the material is electrically conductive.
Concentrations in the range of or above the percolation threshold are necessary for both the antistatic and electrically conductive regions.
In order to be able to establish a surface resistance of about 1 Mxcexa9 for a pigmented polyester material, about 30 to 50% by weight of conductive pigment, based on the resin employed, must be added. Thus, if Minatec(copyright) 30 CM (manufacturer: E. Merck, Darmstadt) is used, a surface resistance of 2 Mxcexa9 is achieved at a pigment content of 30% by weight. Minatec(copyright) 30 CM comprises mica coated with titanium oxide and antimony-doped tin oxide as the conductive layer.
If Sacon(copyright) P 401 (manufacturer: Sachtleben Chemie GmbH, Duisburg), a barium sulfate coated with antimony-doped tin oxide, is used, 50% by weight, based on the resin employed, is necessary to achieve a surface resistance of 20 Mxcexa9.
However, such high pigment contents adversely affect the mechanical properties of the system to be used.
An object of the invention is to provide a conductive pigment which imparts an adequate conductivity to the pigmented system even at significantly below the normal percolation threshold.
Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.
Accordingly, the invention provides a surface-modified conductive pigment which is obtainable by partial coating of a conductive pigment with an organic modifying agent. The partially coated conductive pigment is prepared by dispersing a conductive pigment, subsequently adding the organic modifying agent, if appropriate as a mixture with a solvent, to the pigment dispersion, and in particular in an amount such that the powder electrical conductance measured in xe2x80x9csiemensxe2x80x9d (S)=1/xcexa9, of the partly coated conductive pigment is about 80% or less, preferably 50% or less, of the starting value, but does not fall below a lower limit value of 10xe2x88x927 S, preferably 10xe2x88x925 S, removing the volatile reaction products and the solvent present, where appropriate, with further agitation and heating of the pigment to 100xc2x0 C., and finally drying the partly coated pigment.
The invention also relates to coatings, thermoplastics, casting resins, printing inks and powder coatings pigmented with the pigment according to the invention.
All types of conductive pigments can be employed for partial coating with the modifying agents mentioned below, in particular metallic powders, such as, for example, silver powder and aluminum powder, and furthermore non-metallic powders, such as, for example, carbon black, carbon fibers, antimony-doped tin oxide, aluminum-doped zinc oxide or fluoride-doped tin oxide, and pulverulent substrates which are coated with conductive layers, such as, for example, mica, kaolin or barium sulfate coated with doped metal oxides.
Compounds with organic radicals which can be fixed onto the surface of conductive pigments can be employed as modifying agents. Suitable modifying agents include, for example, organometallic compounds of the elements tin, zirconium, titanium, aluminum and silicon; silanes being preferred. These agents can be used separately or as mixtures.
Particularly suitable compounds are those of the formula I
XR1R2R3R4xe2x80x83xe2x80x83I
wherein the radicals R1-4 have the following meanings:
R1 and/or R2 is alkyl having 1-30 C atoms, and in particular 10-30 C atoms, including cycloalkyl-containing alkyl groups, wherein one or two non-adjacent CH2 groups can also be replaced by xe2x80x94CHxe2x95x90CHxe2x80x94 and/or xe2x80x94Oxe2x80x94, with decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl, octadecyl, nonadecyl, eicosanyl, heneicosanyl, docasanyl, tricosanyl, tetracosanyl, pentacosanyl, hexacosanyl, heptacosanyl, octacosanyl, nonacosanyl, tricontanyl, 12,12-dimethyltetradecyl, 11-propyl-12-butylpentadecyl and 8,8-dimethyl-12-propyl-13-propylhexadecyl radicals being particularly preferred;
the remaining radicals R1-4 (i.e., R2-R4 when R1 is as defined above, R1 and R3-R4 when R2 is as defined above and R3-R4 when both R1 and R2 are as defined above) are halogen or alkoxy having 1-20 C atoms, in particular 1-10 C atoms, wherein one or two non-adjacent CH2 groups can also be replaced by xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94Oxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94COOxe2x80x94 or xe2x80x94OCOxe2x80x94; fluorine, chlorine, methoxy, ethoxy, propoxy, butoxy, pentoxy, hexoxy, heptoxy, octoxy, nonoxy or decoxy radicals are particularly preferred; and
x is Si, Sn, Ti, Zr or Al, the radical R4 being omitted in the case where X=Al.
Particularly preferred compounds are those of the formula
(CnH2n+1)X(OCmH2m+1)p
wherein
X has the above-mentioned meaning,
n is 1-30,
m is 1-10, and
p is 3 when X is Si, Sn, Ti or Zr, and 2 when X is Al.
Among these compounds, n-hexyldecyl-tri-ethoxysilane and methyl-tri-ethoxysilane are preferably employed as modifying agents in the context of the invention.
Silanes are known from EP 0 492 223 for coating pigments. The pigments described therein are used for inhibiting yellowing of plastics.
Partial coating of the conductive pigment is carried out by the process described in DE-OS 40 41 663. For this, the pigment and the modifying agent are mixed intensively in a high-performance mixer at temperatures of preferably about 50-100xc2x0 C., if appropriate with addition of preferably about 1-50% by weight, especially 5-10% by weight, of solvent, based on the pigment. The mixing time is preferably about 1 hour.
If a solvent is used for the coating, then it serves merely for better distribution of the modifying agent on the pigment surface.
If silanes are used, it may be necessary to subject them to preliminary hydrolysis, because the silane does not react directly with the hydroxyl groups on the surface of the pigment. In this case, water and a solvent are added to the silane and the mixture is subjected to preliminary hydrolysis by stirring at room temperature for 30 minutes. After the coating operation, heat treatment of the partly coated pigment, preferably at about 50-200xc2x0 C., is necessary after removal of the solvent.
Water and organic solvents, such as, for example, ketones, ethers and alcohols, are employed for the preliminary hydrolysis. The water and solvent are used in less than the equivalent amount, compared with the amount of pigment to be coated. However, the preliminary hydrolysis can also be carried out without a solvent, as described in DE 40 41 663.
It is decisive for the onset of the effect according to the invention that the surface of the conductive pigment is only partly covered by organic radicals. This partial covering of the pigment surface is achieved by continuing the coating operation until the powder electrical conductance of the partly coated conductive pigment is in the range between 80%, preferably 50%, of the starting value and not less than 10xe2x88x927 S, preferably 10xe2x88x926 S. This means that less covering agent is employed than is necessary to form a complete layer.
Compared with the amount needed for the formation of a complete layer, only 5 to 50% thereof, preferably 10 to 30%, is employed.
The amount of modifying agent depends on the desired conductivity, on the surface of the conductive pigment and on the density of the reactive groups on the surface thereof. The expert is capable of determining the optimum amount for the particular intended use by a few preliminary experiments.
While not intending to be bound by this theory, it is assumed that by partial modification of the pigment surface a certain incompatibility with the binder is effected and the pigment floats to or becomes concentrated on the surface of the coating material or of the coating. Under the influence of the surface tension between pigment and coating matrix, the pigment particles migrate to the surface of the coating matrix and combine there to form a film, which leads to the formation of a network close to the surface. In addition, however, a three-dimensional network also forms in the matrix, resulting in conductivity paths to the surface and within the matrix.
The floating of the pigment to or concentration of the pigment on the surface of the coating agent is called leafing power. It has previously been observed only in metallic effect pigments and described for pearl luster pigments in German Patent Application P 43 23 914.5.
However, because of the system, this property of the modified conductive pigments can be effective during use only if the matrix in which the pigment is employed remains liquid for a certain time without shearing forces simultaneously acting. This is as a rule the case in lacquers, casting resins, coatings, printing inks, powder coatings and the like. The pigments according to the invention can be incorporated into these systems by a known process and then display the properties according to the invention.
The partly coated pigment is stirred, with or without an added solvent, into the matrix, such as lacquer or casting resin, and the components are mixed intensively. The formulation is then applied to the substrate material to be coated (for example, by brushing, dipping, spraying (conventionally or electrostatically) or printing), so that the substrate surface is covered to the desired extent with a homogeneous film. Finally, the coating is dried under the customary conditions.
The leafing power, i.e., the floating of the pearl luster pigments to the surface of the matrix while liquid, and the leafing stability, that is to say unchanged leafing power over a relatively long period of time in the formulation, depend on the matrix used.
This property of the partly modified conductive pigment leads to a significant shift in the percolation threshold and to lower pigment concentrations for achieving adequate conductivity. In optimum cases, this can easily lead to the use of only one third to one tenth of the original percolation concentration.
The degree of pigmentation, the viscosity and the hardening properties of the matrix can likewise influence the leafing effect.
For a good leafing power, the pigmented formulation should preferably comprise not more than about 40% by weight of partly coated conductive pigment. The pigment content is preferably 0.3-30% by weight, in particular 0.5-20% by weight, based on the binder.
Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The following preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.
In the foregoing and in the following examples, all temperatures are set forth uncorrected in degrees Celsius and unless otherwise indicated, all parts and percentages are by weight.
The entire disclosure of all applications, patents and publications, cited above and below, and of corresponding German application P 44 11 807.4, are hereby incorporated by reference.